The present invention relates to method and apparatus for operating an automobile driven by a motor having a fuel cell as an electric power source.
The paper entitled "Propulsion System Strategies for Fuel Cell Vehicles" published in SAE Congress, March 2000 is hereby incorporated for reference.
Internal combustion engines have contributed greatly to the advancement of society. Vehicles powered by these engines have shortened the travel times between us by making long distance road travel routine. Such engines, however, have also greatly contributed to the pollution of the environment. The combustion of petroleum products in these engines results in unwanted byproducts such as carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen dioxide, etc., that are dumped into the atmosphere.
One such alternative energy source is electricity. In general, an electric vehicle comprises an inverter for converting the output of a direct current electric power source into a variable voltage and variable frequency alternating current.
One way to generate electricity for the vehicle is through the use of fuel cells. Fuel cells generate electric energy through the reaction of hydrogen and oxygen. The electric energy that is generated is used to drive an electric motor that, in turn, drives the wheels of the vehicle. The product of the chemical reaction in a fuel cell utilizing hydrogen and oxygen is water, a product which is essential to the environment. Naturally, there is no problem with the disposal of water.
Fuel cells extract energy by electro-chemically reacting hydrogen with oxygen, and are capable of operating for a long time since power is continuously outputted as long as fuel is supplied. The oxygen for the fuel cell reaction can typically be obtained from the ambient air while the hydrogen is obtained, for example, from a hydrogen fuel tank, a hydrogen storage device, or in a reformate stream from a catalytic reformer. The hydrogen and air for the fuel cell are handled by respective air and hydrogen supply systems that are each under the control of, for example, a programmable logic controller.
Fuel cell vehicles can be classified as Fuel Cell Electric and Fuel Cell Hybrid vehicles. A Fuel Cell Electric vehicle uses a fuel cell system as the power source without the use of a battery. Like the battery in the electric vehicles, the fuel cell system provides electricity to the drive train of the vehicle.
A Fuel Cell Hybrid vehicle has a battery or an ultra-capacitor in parallel with the fuel cell system. Fuel Cell Hybrid operation enables the most efficient use of the inherently high energy density of the fuel cell and the high power density of the battery. When power demand is high, such as during acceleration, batteries provide the required power. When the power demand is low, such as during cruising, the fuel cell provides the required power. Batteries are recharged during the periods of low power operation. Thus, depending on the power and energy requirements, the fuel cell could be designed to provide cruising power and the battery could be designed to provide peak power. The selection of the battery pack would also depend on factors such as cost and performance of the fuel cell and the battery, the battery technology, and the driving cycle. The use of a battery allows rapid start-up of the fuel cell and protects it against cell reversal during operation.
In addition, the battery provides peak power, regeneration energy that can be captured, and the response time of the vehicle system for load changes is faster with the battery. A fuel cell hybrid offers good performance, long range, fast refueling, and long life.
A fuel cell vehicle system consists of three main components. The first component is a fuel processor where the fuel is converted to a hydrogen-rich gas. The second component is a fuel cell power section which consists of a stack of fuel cells where the hydrogen gas and oxidants are combined to produce direct current electricity. The third component is a heat power inverter/converter to convert the fuel cell power to ac or dc depending on the nature of the load.
If the vehicle is not operated with hydrogen, then a methanol or a gasoline reformer with a storage tank to store the fuel is needed to generate hydrogen on-board. The fuel cell, peak-power devices, motor, electronics, and fuel storage system must be designed and arranged to fit into as small a space as possible, without creating safety hazards.
One of the important parameters for the total overall efficiency of the vehicle is how fast the fuel cell system can respond to a change in the power demand. The energy consumption for the vehicle depends on the response time of the fuel cell system. Any fuel consumed during the start-up will have a negative effect on the fuel economy. If the fuel is direct pressurized hydrogen gas, a very fast response can be obtained. If an on-board methanol reformer is used, the response time is considerably longer. The response time is determined by how fast the fuel and air can be supplied to the fuel cell. The response time can vary considerably depending on the type of fuel used, type of the reformer and its construction.
The methanol reformers take a relatively long time to warm up, and cannot follow rapid changes in power demand. The response time can be up to one minute. The technology is in the development stage to have reformers with a response time of less than 100 ms. In order to decrease the response time of the reformer based system, a buffer hydrogen tank can be used to provide the required fuel during transient conditions.
However, the additional hydrogen tank adds cost, weight, and control complexity to the vehicle. Also, the sizing of the hydrogen tank could be a challenging problem. The exact amount of hydrogen needed depends on the operating conditions. It is important to make sure that there is enough hydrogen when the vehicle is accelerated. Currently, there is no infrastructure for refilling hydrogen.
Research is going on in the areas of direct methanol and direct gasoline based fuel cell stack systems. Direct reduction of methanol or gasoline in the stack reduces system complexity. It enables quick start-up for temperatures greater than 0.degree. C. Still more development work is needed to remove carbon monoxide from the fuel cell stack. Further, the efficiency of the direct hydrogen stack is superior to a fuel cell stack that obtains hydrogen from a reformer.
A fuel cell system designed for vehicular propulsion applications must have weight, volume, power density, start-up, and transient response similar to the present day internal combustion engine based vehicles. Other requirements are: very high performance for a short time, rapid acceleration, good fuel economy, easy access and safety considerations with respect to the fuel handling. Cost and expected life-time are also very important considerations.
It would, therefore, be advantageous to have a propulsion control system where the fuel cell is controlled in a quick and efficient manner.
Therefore, it would be desirable to provide an improvement, which overcomes the aforementioned inadequacies of the prior art and provides a significant contribution to the advancement of the art of fuel cell vehicle propulsion.
Accordingly, what is needed in the art is an improved fuel cell propulsion system that is capable of responding to a wide range of output power requirements, and driving a long travel distance, by combining a storage battery and a fuel cell in an optimum usage pattern, taking full advantage of both the fuel cell and the storage battery under various operating conditions.
It would be further desirable to provide a vehicle driving system having a fuel cell power source which is reduced in size and cost, and where the fuel cell is characteristically matched to the power battery thereby eliminating the need for a DC/DC boost converter.
It would also be desirable to provide a fuel cell propulsion system where a standard 12 volt battery is used to start the vehicle through a buck/boost converter thereby eliminating the need for a power battery, and where the fuel cell unit produces a compatible voltage to the propulsion motor and a standard 12 volt battery is used to start the vehicle through a buck/boost converter thereby eliminating the need for a DC/DC boost converter and the need for a power battery.